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GH-39669: [C++][Gandiva] Ensure Gandiva benchmarks present a bytes/s or items/s metric (#40435)



### Rationale for this change

The Gandiva microbenchmarks only present an iteration time in (nano,micro...)seconds. That is usually tedious to read and difficult to interpret.

### What changes are included in this PR?

Ensure that Gandiva benchmarks present an items/seconds and/or a bytes/seconds metric where that makes sense.

### Are these changes tested?

Manually

### Are there any user-facing changes?

No

* GitHub Issue: #39669

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>